patchrebase: emit one "fatal" in "fatal: fatal: <error>"

>
> The die() routine adds a "fatal: " prefix, there is no reason to add
> another one. Fixes code added in e65123a71d0 (builtin rebase: support
> `git rebase <upstream> <switch-to>`, 2018-09-04).
>
> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> ---
>  builtin/rebase.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index 33e09619005..66a0a0f0d03 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -1918,7 +1918,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
>                                                    &options.orig_head))
>                         options.head_name = NULL;
>                 else
> -                       die(_("fatal: no such branch/commit '%s'"),
> +                       die(_("no such branch/commit '%s'"),
>                             branch_name);
>         } else if (argc == 0) {
>                 /* Do not need to switch branches, we are already on it. */